In information processing systems, the trend in technology is to use magnetooptical disk memories, because of the large amount of information they are capable of containing per unit of surface area. These are memories in which the information is recorded on magnetic disks (for example by means of magnetic trandsducers) and read by optoelectronic devices.
Their mode of operation is based on the magnetooptical effect, which has to do with the interaction of a rectilinear polarized light with the magnetic state of the material comprising the recording layer of the magnetic disks. Reading of the information is performed by an optoelectronic device, including a more or less complex optical focusing device associated with photoelectronic transducers and amplifying circuits for the signals furnished by these transducers. These optoelectronic devices make it possible, at a given moment and in a given region, to observe a surface of a disk by means of a beam of polarized light, and to furnish an electrical signal the voltage (or current) of which is a function of the value of the information located in this region. The magnetooptical effect is described in greater detail, and the manner in which it can be used to read the information contained on the magnetic disks of the magnetooptical memory is also discussed in French Patent 2 514 913, filed on Oct. 16, 1981 by CII Honeywell Bull, now known as Bull S.A. This French patent corresponds to U.S. Pat. No. 4,510,544.
It is known that the magnetic disks carry these items of information in an encoded binary form on circular concentric recording tracks the width of which is on the order of several micrometers and which are disposed on both surfaces of the disks.
Each track is assigned a serial number j, j being an integer varying from 0 to N--1 and N being the total number of recording tracks. This number of tracks is on the order of several thousand. The encoded expression of the serial number j of a track is known as its address. In this case, the address is called the "absolute address".
The magnetic disks have a constant speed of rotation.
In practice, with standard disk memories (where the information is written and read by the same magnetic transducer), and more particularly in the case of memories that include only a limited number of disks (generally fewer than 4 or 5), the information is recorded on each of the surfaces (sides) of the disks in the manner described in French Patent 2 439 435, filed on Oct. 19, 1978, and corresponding to U.S. Pat. No. 4,354,208. A maximum of space is reserved for recording the data intended for processing by the information processing system to which these memories belong.
A minimum of space is reserved for recording the addresses of the tracks, on the one hand, and on the other for recording the information, known as "fine-position information", necessary for the automatic control of the position above the tracks of the magnetic transducer associated with this side.
In present practice, as described in the aforementioned French patent, the information contained on each side of the disk is preferably distributed over equal and adjacent circular sectors S.sub.0, S.sub.1, . . . , S.sub.i, . . . , S.sub.n. Typically, one side of the disk is divided into several tens of sectors (for example on the order of 80 to 90 sectors).
When a sector S.sub.i (or more generally a first group of information) is read or written prior to a sector S.sub.i+1 (or more generally, a second group of information), then it is said that the sector S.sub.i precedes the sector S.sub.i+1.
Each sector S.sub.i is in turn divided into two unequal areas. The larger area includes the data intended for processing by the information processing system to which the disk memory belongs, while the smaller area includes the track addresses and the fine-position information. For each sector, the smaller area is divided into a plurality of zones known as reference zones. Each track is associated with at least one zone having the same serial number j as the track.
A blank zone not containing any information is disposed between the larger area and the smaller area. This blank zone precedes the reference zones.
It is known that in order to record a succession of information on a magnetic disk, a succession of small magnetic domains adjacent to one another and of variable length are created on each track of the disk, distributed over the entire length of the track and alternatingly having magnetization of one type and the opposite type. The geographic boundary between two adjacent magnetic domains is called the magnetic transition.
Thus as described in French Patent 2 439 435, the reference zones have the same width as the tracks, each zone being offset by a distance equal to the width of one-half of a track, with respect to the track having the serial number j with which it is associated. Because of this, the boundary between two adjacent reference zones of serial numbers j and j+1 is coincident with the middle of the track having the serial number j.
Moreover, each reference zone includes three portions, that is, a first portion known as the preamble, preceding a second portion containing address information, which in turn precedes a third portion including the fine-position information. The preamble portion contains the information, the use of which by the reading circuits of the disk memory makes it possible to determine the gain of the amplifiers of these circuits such that the precision of reading the addresses and the fine-position information is as great as possible. This preamble information can equally well serve as synchronizing information making it possible to determine the beginning of each reference zone.
The address information is written using the Gray code; that is, two successive addresses written in two adjacent reference zones associated with tracks of serial number j and (j+1) differ by only a single bit. Preferably, the three aforementioned portions are of the same length and include the same number of cells, each datum comprising the presence or absence of a double magnetic transition.
The writing mode described briefly above, used in conventional disk memories, can be transposed and applied to magnetooptical disk memories on the condition that the following disadvantages are overcome:
When the reading transducer reads the data recorded on a predetermined track, being perfectly centered over the track, it reads the information contained in the reference zones, straddling them; that is, it simultaneously reads the information contained in each of these zones. Since the addresses are written in Gray code, the address information read by the reading transducer will all be determined perfectly, except for a single bit, known as the uncertain bit, since two adjacent addresses differ by this bit. Hence it is permanently necessary to determine the value of this uncertain bit with a supplementary electronic circuit, which lends a relative complexity to the electronic reading circuit.
Since the items of fine-position information are relatively numerous, the associated electronic circuits that make it possible to determine whether the writing or reading transducer is perfectly centered over the track that is being written upon or read are relatively complicated.
The portion containing the preamble information is poorly adapted to use in magnetooptical disk memories. In fact, the disks used in these memories have an error rate on the order of 10.sup.-5 (one error per 10.sup.5 items of information written), which is considered relatively high. The first portion containing the preamble information simultaneously serves on the one hand to monitor the gain of the reading circuit amplifiers and on the other to determine the beginning of the zone and so does not offer sufficient warranty for precise detection of the reference zone, given the aforementioned error rate.